This invention relates to intrusion monitoring devices, and more particularly to an improved voltage level detection circuit for an intrusion monitoring device having reduced sensitivity to noise and interference.
Commercially available intrusion monitoring devices can be either of the passive or active variety. Passive intrusion monitoring devices can, for example, comprise a sensor which detects infra-red radiation propagated by warm blooded animals. Typically such passive devices comprise a thermal detection device, consisting of one or more thermal detectors adapted to detect infra-red radiation incident thereon, and an optical system for directing incident radiation from a plurality of angular fields of view towards the thermal detection device. Such optical systems may consist of lenses, particularly Fresnel lenses and/or reflecting surfaces. Normally such devices are activated when a source of infra-red radiation passes from one angular sector to the next. Typical prior art intrusion monitoring devices are illustrated in U.S. Pat. Nos. 3,703,718 and 3,958,118, and in UK Patent No. 1335410. A Fresnel lens for use in an intrusion monitoring device is described in U.S. Pat. No. 4,787,722. The complete disclosures of all these patents are included herein by reference.
Active intrusion monitoring devices are also known which comprise a transmitter and a receiver, the transmitter emitting radiation at a defined frequency and the receiver measuring the Doppler shift in any reflected radiation. Such active devices can, for example, operate at microwave frequencies, using a microwave radiation detection device to detect the reflected radiation. Whatever detection device is used, it is necessary in each case to provide an electrical circuit to process the electrical output signal of the detection device and to compare that signal with a pre-set threshold signal.
A problem which has, however, been encountered with conventional intrusion monitoring devices is that if the signal from the detection device has noise or interference superimposed upon it this can result in a false reading leading to an event being incorrectly registered.
It is an object of the present invention to provide an intrusion monitoring device in which the electrical circuit used to process the signal from the detection device is provided with means for suppressing false readings caused by noise and interference superimposed upon the signal.